Service bushing test

ABSTRACT

Apparatus for measuring a defect characteristic of a bushing which has a high voltage electrode passing through a grounded flange, insulating material around the electrode and a capacitance tap, while the bushing is in service connected to a high voltage source, includes an impedor connected permanently between the capacitance tap of the bushing and ground. The impedance of this impedor is so low relative to the impedance of the insulating material of the bushing between the capacitance tap and ground that the voltage between the capacitance tap and ground does not exceed one hundred volts. A pluggable connection permits an external circuit to be coupled to the impedor for providing an indication of a defect characteristic of the bushing.

United States Patent n91 Povey [451 Jan. 9, 1973 [54] SERVICE BUSHINGTEST [75] Inventor: Edmund H. Povey, Medford, Mass.

[73] Assignee: Doble Engineering Belmont, Mass.

[22] Filed: Feb. 12, 1971 [2]] Appl. No.: 114,780

Related U.S. Application Data [63] Continuation-impart of Ser. No.26,541, April 8,

1970, abandoned.

Company,

[52] U.S. Cl. ..324/54, 324/60 R [51] Int. Cl. ..G0lr 3l/l 2, GOlr 27/26[58] Field of Search ..324/60, 83 A, 126, 54

[5 6] References Cited UNITED STATES PATENTS 2,307,499 l/1943 Frakes..324/60 X Primary Examiner-Alfred E. Smith Attorney-Willis M. Ertman[57] ABSTRACT Apparatus for measuring a defect characteristic of abushing which has a high voltage electrode passing through a groundedflange, insulating material around the electrode and a capacitance tap,while the bushing is in service connected to a high voltage source,includes an impedor connected permanently between the capacitance tap ofthe bushing and ground. The impedance of this impedor is so low relativeto the impedance of the insulating material of the bushing between thecapacitance tap and ground that the voltage between the capacitance tapand ground does not exceed one hundred volts. A pluggable connectionpermits an external circuit to be coupled to the impedor for providingan indication of a defect characteristic of the bushing.

19 Claims, 9 Drawing Figures PATENTEUJAN 9 I973 sum 1 0F 3 FIG.

FIG. 2

FIG 3 PAIENIEDJM 9197s 3.710 242 SHEET 2 OF 3 FIG '5 SERVICE BUSHINGTEST This application is a continuation-in-part of my copending patentapplication Ser. No. 26,541 filedApr. 8, 1970 now abandoned, entitledMethod and Apparatus for Measuring the Capacitance and Defect Angle in aHigh Voltage Bushing."

This invention relates to apparatus for testing bushings while they arein service. Such bushings are used with high-voltage electricalapparatus as, for example, transformers and oil switches, and theyusually consist of a cylindrical high-voltage electrode passing througha grounded flange fastened to the apparatus housing, and insulated withsheet insulating material wrapped about the conductor and enclosedwithin a protective porcelain shell. For use at the higher voltages,these bushings are often oil-filled..Metallic foil interwrapped betweenthe layers of insulating material provides a number of series capacitorsbetween the high-voltage electrode and ground, which serves todistribute the electrical stress nearly uniformly within the insulation.

During service, the insulation of such bushings deteriorates and unlesscorrective measures are taken, failure may occur, resulting in theinterruption of power service and damage to the electrical equipment.

.Thus, it is important to test such bushings periodically to detectdeterioration before failure occurs. Obviously, it is highlyadvantageous to be able to make such tests without removing the bushingsfrom service, thus. avoiding making changes in system-switchingarrangements, as well as any interruptions of the powerin the apparatusassociated with the bushings.

In order to test a bushing that is in service, the bush ing must beequipped with a test electrode, and highvoltage bushings are furnishedwith a capacitance tap which can be used as a test electrode. Thecapacitance tap consists of a connection to a layer of metallic foilnear the outer wrap of the insulation material, which connection isbrought out to a terminal near the flange of the bushing. The majorinsulation of thebushing lies between the center conductor and thecapacitancetap layer.

The capacitance tap may be connected to a potential device foractivating relays or instruments, but in most bushings, it is not used.It may be grounded to the flange or may be left open-circuited andcovered with a protective cap. On opencircuit, the tap can have avoltage in the order of several thousand volts.

In order to use the capacitance tap for test purposes, previous testingmeans required the removal .of any other tap connection, such as aground or the lead to a potential device. Since the open-circuit voltageof the tap is sufficiently high to be a potential danger topersonnelmaking the test connection, protective measures must be used.

Accordingly it is an object of the invention to provide novel andimproved methods and apparatus for measuring characteristics of highvoltage bushings while they are in service.

Another object of the invention is to provide novel and improved meansfor the measurement of the capacitance and defect angle of the maininsulation of high voltage bushings, while they are in service, with.apparatus that is easily and safely installed. and operated and providesaccurate readings.

Another object of the invention is to provide novel and improvedmeansfor the detection of corona or partial discharge damage in the highvoltage bushings while they are in service and with apparatus that iseasily and safely installed and operated and provides accu ratereadings.

Another object of the invention is to provide novel and improved highvoltage bushing arrangements to which test apparatus may be connectedand disconnected readily.

A still further object of the invention is to provide a novel andimproved high voltage testing arrangement which maintains the potentialof the capacitance tap at a safe, low value and does not require removalof the bushing from service for testing.

According to methods and apparatus of the invention, a defectcharacteristic: of a high voltage bushing which has a conductor passingthrough a grounded flange, with insulating material around the conductorand a capacitance tap, is measured while the bushing is in serviceconnected to a high voltage source, through the provision of an impedorconnected permanently between the capacitance tap of the bushing andground, and adapted to be coupled to an external circuit. The impedanceof the impedor is so low relative to that of the insulating material andto that of the bushing between the capacitance tap and ground that thecurrent in the impedor is determined. almost solely by the impedance ofthe insulating material. The nature of the external measuring circuitdepends on which type of measurement is made. When measurement of defectangle and capacitance is desired, the external circuit utilizes a lowreference voltage having known voltage ratio and phase angle relation tothe high voltage source. The apparatus produces an output voltage as afunction of the current flow in the impedor and includes means tocompare the output voltage in both magnitude and. phase with thereference voltage. The external measuring circuit used to detect coronaor partial discharge in the bushing utilizes a frequency selectiveamplifier to amplify those frequencies of interest and anappropriatemeasuring device to observe their relative strength. The impedor and theexternal circuits are arranged to be connected to each other bydetachable leads using a polarized connector.

In preferred embodiments of apparatus for measuring capacitance anddefect angle, the apparatus has connecting means to permit the couplingof the external measuring circuit to the impedor without interruption ofthe current through the bushing, amplifying means in the externalmeasuring circuit including phase shifting means, preferably calibratedin terms of bushingpower factor, to adjust the phase of an outputvoltage in conformity with the phase of a reference voltage which isestablished at a remote location, magnitude control means to cause theoutput voltage to attain magnitude equality with the reference voltage,and shielded :leads to conduct the reference voltage to the externalmeasuring circuit for comparison with the output voltage.

In one such embodiment of the invention the impedor is a fixed capacitorconnected permanently between the capacitance tap and ground, thereactance of the capacitor being so low that current therethrough isdetermined almost solely by the impedance of the in-.

sulating material and the voltage between the capacitance tap and groundis maintained at less than I volts. The capacitor provides a voltageresponsive to the current through the insulating material. Amplifyingmeans with negligible phase shift and an adjustable gain controlprovides an output voltage in response to the voltage from thecapacitor. The low reference voltage is applied to a potentiometer withan adjustable contactor and means are arranged to indicate when thevoltage difference between the output voltage and the voltage at thecontactor attains a minimum value. Another embodiment further includesan adjustable second capacitor, connected in parallel with the firstcapacitor and calibrated in terms of the capacitance of the bushing,phase shifting means to adjust the phase of the output voltage toconformity with the phase of the low reference voltage, and controlmeans to cause the output voltage to attain magnitude equality with thereference voltage.

In another embodiment of the invention the impedor is a permanentlyconnected resistor of low value. The apparatus also has adjustableresistive control means arranged to relate the capacitive component ofthe output voltage to the capacitance of the bushing, phase shiftingmeans arranged to cause the phase of the output voltage to conform tothe phase of the low reference voltage, adjustable resistive control andamplifying means arranged to cause the magnitude of the output voltageto conform to the magnitude of the low reference voltage, and detectingmeans to compare the output voltage with the reference voltage.

In a further aspect of the invention, for the detection or measurementof partial discharges, an inductive impedor is included in the permanentconnection between the capacitance tap and ground. The impedor has a lowreactance at the power frequency so that the tap voltage, due to thehigh-voltage power system, is never in excess of 100 volts. Partialdischarges produce voltages across the inductive impedor which containcomponents of much higher frequency than that of the power system. Thehigh frequencies are amplified by a frequency-selective amplifier andsensed, for example, by display on a cathode ray oscilloscope whereinthe relative amplitudes of the output signal pulses are evaluated, or bya peak reading meter or by separation of frequencies with a high passfilter, rectification and reading on a DC instrument.

In a preferred embodiment of the invention, resistive and inductiveimpedors are connected in series and the two impedors are permanentlyconnected between the capacitance tap and ground. In a first mode theinductive impedor is short circuited by the connecting means and theresistive impedor is used to measure capacitance and defect and while ina second mode, the resistive impedor is short circuited and theinductive impedor is used to sense partial discharge defects.

The invention thus provides methods and apparatus for accuratelymeasuring defect characteristics of a high voltage bushing while thatbushing is in service; and is particularly useful in facilitating themeasurement of bushing capacitance and defect angle, and/or partialdischarge defects.

Other objects, features and advantages will become apparent from thefollowing description of particular embodiments of the invention, takentogether with the attached drawings thereof, in which:

FIG. 1 is a schematic diagram of a bushing and testing apparatus inaccordance with the invention;

FIG. 2 is a phasor diagram representing the voltages and currents forthe circuit of FIG. 1;

FIG. 3 is a perspective view, showing adjusting dials and the indicatingneedle, of the assembled testing apparatus in a metallic container;

FIG. 4 is aschematic diagram of another embodiment of the invention inwhich resistive and inductive impedors are employed;

FIG. 5 is a phasor diagram of the voltage and currents of the circuit ofFIG. 4 (with the inductive impedor short circuited) before the system isadjusted to measure the capacitance and phase angle of the bushing;

FIG. 6 is the phasor diagram of FIG. 5 after the adjustment has beenmade;

FIG. 7 is a schematic diagram of the bushing and impedor arrangement ofFIG. 4 with the resistive impedor short circuited and another externalcircuit connected;

FIG. 8 is a schematic wiring diagram of still another embodiment of theinvention; and

FIG. 9 is a phasor diagram of the system of FIG. 8 after adjustmentshave been made.

With reference to FIG. 1, a typical bushing 10 has alternate layers ofmetallic foil and sheet insulating material which may be representedschematically as sectional series capacitors 12, 14, 16, and 18connected between the high voltage bushing conductor 20 and ground 22.As will be readily understood by those skilled in the art, the actualnumber of capacitors in a bushing may be much greater than the fourshown in this illustration.

The capacitors are shown diagrammatically as being shunted by resistors24, 26, 28, and 30, respectively, which represent the dielectric andleakage losses in the bushing insulation. A capacitance tap 32 isbrought out from between the ground capacitor unit 18 and the unit 16.Although a single capacitor unit 18 is shown between the capacitance tap32 and ground 22, actually it may consist of several layers of metallicfoil and insulation in series.

Thus, the bushing may be represented simply by two capacitances, C, thecapacitance of the main insulation between the high voltage terminal andthe capacitance tap 32 (that of capacitors 12, 14, and 16, in series),and the capacitance C of the insulation between the capacitance tap 32and ground 22 (that of capacitor 18).

A first embodiment of the invention is illustrated by FIG. I in which animpedor Z in the form of a low reactance capacitor 34 is permanentlyconnected in parallel with capacitor 18 between tap 32 and ground 22.Leads 36 and 38 are provided to permanent female connectors 40, 42,which are arranged to permit connection to external circuitry and whichmay be, for example, in a polarized plug insulated from the housing ofthe high voltage bushing by additional small insulated bushings. Thebushing 10 is connected between one conductor 20 of a high voltage powerline and ground 22, the conductor operating at E;. volts to ground.

Capacitor 34, connected permanently between capacitance tap 32 andground as shown, limits the voltage at the capacitance tap to a lowvalue, the magnitude of the impedance of impedor Z being chosensufficiently low in comparison with that of C so that practically theentire current I, from C, flows through 2, and sufficiently low incomparison with that of C, so that it may be ignored in determining thecurrent through C, due to line voltage E,.. In a l lS-kV (66-kV toground) bushing, a nominal value of C, is 330 picofarads, which at afrequency of 60 Hz gives an impedance of about 8 megohms. The nominalcapacitance C, of such a bushing is 2,700 picofarads, which at afrequency of 60 Hz gives an impedance of about 1 megohm. For theforegoing 66-kV bushing, an impedor Z having an impedance of I00 ohms orless meets the requirements.

An external measuring circuit, shown connected across impedor 34 atconnectors 40 and 42 by leads 44 and 46, respectively, includes highgain amplifier 48 (for example, Analog Devices, Inc., Model 1 18) havingpositive input terminal 50 connected to lead 44, output terminal 52connected to ground 22 through potentiometer 54, and negative inputterminal 56: connected to contactor 58 on potentiometer 54. Thus, anegative feedback network is formed, and since the gain of amplifier 48is of the order of 105, the amplification factor, k, of the network isdetermined by the position of R, ohms from ground. Potentiometer 62 hasone side.

grounded and is connected over long shielded. leads 66 to the referencevoltage, which in this case appears across the secondary winding 68 of apotential transformer 70. The primary winding 72 of potentialtransformer 70 is connected between high voltage conductor 20 and ground22. For purposes of illustration the voltage between conductor 20 andground 22 is referred to as E,,, that across impedor 34, as.E,, thatacross potentiometer 54 as E,, that across secondary winding 68, as thereference voltage F,, and that between contactor 60 and ground 22 as Es.Reference voltage E, is in phase with E, and in a known ratio, E,thereto and is obtained from the secondary winding 68 of potentialtransformer 70. E must have a known ratio to E, and can be either inphase with it or have a known phase angle relation thereto. Thereference voltage alternatively may be obtained from a bushing connectedto the same high voltage line 20 and whose capacitance and phase angleare known or from the apparatus disclosed in U.S. Pat. Nos. 2,922,952and 3,272,051, both of which are issued to E. H. Povey and C. L. Dawes.The potential transformer or other apparatus, which supplies thereference voltage, is often located in the switchyard some distanceremote from the bushing under test. Accordingly, long shielded leads 66may be necessary to conduct the reference voltage E, to the outputvoltage Es for comparison. Such leadsshould produce no significantattenuation or phase shift in the signal and they should also beshielded to exclude any interference from stray electric and magneticinduction that may be present.

The operation of the system is best understood by an analysis of thecircuit voltages and currents. Because of the low impedance of theimpedor Z (34) relative to that of section C, of the bushing,.thebushing current I, is given with a high degree of accuracy by thefollowing equation:

voltage E, the angle (--a) is referenced from the line voltage E,,, anda is the defect angle of C,, a measure of the loss characteristic of C,.The current 1, leads E, by (90a) as shown in FIG. 2, which is a phasordiagram of the system. For clarity the diagram is not drawn to scale.The current I, divides at tap 32, only an insignificant amount going toground 22 through C because of its high impedance relative to that of Z(34). The current goes to ground 22 through Z (34), practically noneentering terminal 50 of amplifier 48 since the latter has almostinfinite input impedance.

The voltage E,, at the positive terminal 50 of amplifier .48 is E,=kE, I4

k being the amplification factor of amplifier 48, as determined by thecontactor setting of potentiometer 54.

The reference voltage E E /K, 5

where K is the known potential transformer ratio.

The voltmeter 64 measures the voltage difference between the amplifieroutput terminal 52 at voltage E, and the contactor 60. Contactor 60 ismoved along the potentiometer 62 until the reading of voltmeter 64reaches a minimum value E shown in the phasor diagram, FIG. 2. A studyof FIG. 2 shows that while the contactor 6 0is being moved along thepotentiometer, the voltage measured by the voltmeter 64 will varybetween such limits as are indicated by the dotted lines originating atE,. It is obvious that when the voltage difference between E andcontactor 60, measured by voltmeter 64, reaches a minimum value E,,, Ebecomes perpendicular to E,, and the corresponding voltage at thecontactor becomes E s. Then M tan a '"E,,/ Es 6 sin: a E /E, I 7 Todetermine the bushing capacitance C note that C,.and C form a capacitordivider (neglecting the effect of C C, is small compared with C that itmay be neglected. Hence (8) becomes From FIG. 2,

E,=E, cos a E, (R,/R cos a 10 where R, is the total resistance of thepotentiometer and R,is the amount of resistance to ground tapped off bythe contactor 60 to provide voltage E The angle a is so small that cos ais essentially unity, therefore,

Combining (4), (5), (9) and (1 l the capacitance,

C R C /kK R 12 The values of R R k, K, and C are known or may bedetermined easily, so that the apparatus provides a simple, yetaccurate, method for determining the value of C Furthermore, themeasurements may be taken while bushings are in service and withoutopening the tap connections and the measuring circuit can convenientlyconsist of a portable self-contained test set 76 (FIG. 3) which may bereadily and safely plugged into the bushing test connections.

Another embodiment of the invention is shown in FIG. 4. This embodimentincludes a resistive impedor 80, with no appreciable phase shift,connected in series with an inductive impedor 81 between tap 32 andground 22. Leads 36 37, and 38 are brought out to female connectors 4140, and 42.

When a first test connection is made as shown in FIG. 4, inductor 81 isshorted by jumper 82 and the .capacitance tap 32 is connected to thenegative terminal 83 of operational amplifier 84 (for example, AnalogDevices, Inc., Model 118) through a fixed resistor 86 and a variableresistor 88, which preferably is a multi-turn device. The positiveterminal 90 of operational amplifier 84 is connected to ground 22 andfeedback capacitor 92 is connected between the negative terminal 83 andthe amplifier output terminal 94. A potentiometer 96, preferably with amultiturn dial, is connected between output terminal 94 and ground 22,its contactor 98 being connected to the negative terminal 83 ofamplifier 84 through a fixed resistor 100. Resistor 100, capacitor 92,and potentiometer 96 comprise a phase-shifting circuit designed toproduce a small phase shift in E the output voltage of the system. Thisphase-shifting circuitry is advantageous since it provides a controlwhich can be given a linear calibration in terms of an angle [3(Equation l6 below) and when [3 is small, such as is usual withbushings, the multi-turn dial on potentiometer 96 can be calibrateddirectly in terms of power factor. In contract, the usualresistor-capacitor, phase-shifting circuit requires an inversecalibration. As is well known to those skilled in the art, operationalamplifiers have a very high internal gain, so that for normal outputvoltages, the potential difference between the two input terminals is sosmall that it usually can be neglected. Also, the input impedance ofoperational amplifiers is usually so high that practically no currentflows into their input terminals.

The output terminal 94 of amplifier 84 is connected through a fixedresistor 102 and variable resistor 104, preferably of the dial operatedtype, to the negative terminal 106 of a second operational amplifier 108(for example, Analog Devices, Inc., Model 1 18). The positive terminal110 of amplifier 108 is connected to ground 22 and a feedback resistor112 is connected between negative terminal 106 and the output terminal114. The magnitude of output voltage E can be controlled by varying theresistance of resistor 104.

For the purpose of illustration, reference voltage E, is supplied by thesecondary of a potential transformer 116 whose primary is connected tothe high-voltage conductor 20. As will be shown later when variableresistor 104 has been set in accordance with the known ratio E /E, K,and the variable resistor 88 and potentiometer 96 have been adjusted tomake output voltage E equal in phase and magnitude to E,, thecapacitance and defect angle (or the power factor) of the maininsulation of the bushing C may be read from appropriate dials onresistor 88 and potentiometer 96, respectively. It should be noted thatthe functions of the variable resistors 104 and 88 may be interchangedor combined in either resistor; the arrangement shown is a convenientone.

The condition noted above, that is when the output voltage E has beenbrought to equality with the reference voltage E,, is indicated by thenull detector 120 (for example, a millivoltmeter) connected between theoutput terminal 1 14 of amplifier 108 and the output terminal of thesecondary of the potential transformer 1 16 (through shielded leads 118).

The development of the output voltage E from the bushing current 1 willbe followed through the circuit. The current I through C may berepresented to a high degree of accuracy of Equation 1:

I =E wC,at angle (90a), l

Current I divides at tap 32, an insignificant amount proceeding toground through C part (1 proceeding to ground through resistor 80, andpart (1 proceeding to the negative input terminal 83 of the amplifier84. Since positive input terminal is grounded, and since no significantpotential difference normally exists between the negative and positiveterminals, negative input terminal 83 is at virtual ground potential.Under this condition, the relation between I and 1 is as follows:

Since the r a tio in Equation 13 is numeric, 1, is in phase with 1 (FIG.5).

Currents arriving at negative input terminal 83 of amplifier 84 must addto zero, since no appreciable current flows into the input terminal.Therefore,

7 Amplifier 8 4 is required to produce an output volti 1 5, 5.7,, 1, or1,, =1, l4

age E which will supply the required current 1 Assuming for the momentthat the contactor 98 of potentiometer 96 is at ground potential asshown in FIG. 4, and recalling that negative input terminal 83 is atvirtual ground potential, then l =0 and 1 1 The voltage E (noting therelation expressed by (14)) must therefore be B 92) l6 so thatexpression 15 becomes l t. [Edi E 1,0,, 17

Substituting 1 from l 3),

so ae Raa n:

The current into the I second amplifier I is produced by the outputvoltage of the first amplifier 84 flowing through resistors 102 and 104.Here again, the positive input terminal 110 is at ground potential sothat negative input terminal 106 is at virtual ground. potential. Thecurrent flowing toward this terminalis |o2 1a/-( roz+ w4) The outputvoltage E of the amplifier 108 must produce an equal but oppositecurrent through resistor 112. The output voltage must be a o 21 1o2+ 104/180 Substituting E from 1 9 thbut' itit voltage becomes Voltage E canbe varied in mag nitiid e by adjii sting R the defect angle can bevaried by adjusting R and the capacitance can be varied by adjusting Rall Ratio (K 1o2+ ro4) 24 where B is a constant. Note that the lowestdial reading is a value of K =BR which occurs when R m is equal to zero.

The dial on R is made to read in terms of the angle B. In the discussionleading to the development of ex- 'pression (16), it was pointed outthat current ean be varied from -1 percent of 1 if contactor 98-ofpotentiometer 96 is moved from one extreme \to the other. The angle ,8is a function of the ratio of 1 to I as given by expression (16) whichin the usual bushing is very small.

To operate the measurement system in order to obtain the capacitance anddefect angle a of the bushing 10, first:

the ratio dial (104) is set for the knownratio K. The capacitance dial(88) and the defect angle dial (96) are adjusted simultaneously untilthe output voltage E, is

equal to the standard reference voltage E, in phase and magnitude, asindicated by null detector 120. Obviously, to obtain zero angle, thedefect angle control potentiometer (96) must be set :so that its angle Bis equal to a the defect angle of the bushing, then Under theseconditions, using the relations (21), (22), (24), and (25), which givesE,,(AR /Cap. Dial) (BRm/K) cg/c9.) Et/K 26 from which I Cap. Dial c ABRQMI W) 27 It willbe seen that by selecting proper values for A, B, R Rand C that the last term in expression 27 can be reduced to unity andthe capacitance dial will read the capacitance C of the main insulationof the bushing.

The phasor diagram, FIG. 5, which for clarity is not drawn to scale,shows the relations among the voltages and currents of the circuit ofFIG. 4 before the adjust ments of capacitor dial 88, the ratio dial 104,and the phase potentiometer 96 dial have been adjusted to bring E intophase and magnitude equality with E,. The bushing current 1, leads thevoltage E,, of the highvoltage conductor 20, by the angle 0!), where ais the defect angle of the bushing (Equation (1)). Since Z is resistive,the voltage E; is in phase with I, and as no adjustment of potentiometer96 has yet been made, its c ontactor 98 is at ground potential and 1 ispractically zero. From Equation (14), -I and I =I as shown. Capacitor 92is connected between the negative terminal 83 of amplifier 84 and itsoutput terminal 94, negative terminal 83 being virtually at groundpotential. The voltage E at terminal 94 must lead I by 90 (Equation(15)) (B 0, since no phase adjustment has yet been made). It thus leadsE;, by a), in accordance with Equation (19) with ,6 0. In accordancewith Equation (21 the phase of output voltage E is 180 with respect tothe E and lags E by the angle a in accordance with Equation (22) inwhich B is zero.

FIG. 6 is a phasor diagram of the voltages and currents of FIG. 4 duringthe time that the adjustments are being made and after their completion.As stated earlier, to operate the measurement system, the ratio dial 104is set for the known ratio K, and the capacitance dial 88 and the defectangle dial 96 are adjusted simultaneously. Setting the ratio dial 104changes the mag nitude of E and the simultaneous adjustments of thecapacitance dial 88 and the defect-angle dial )6 (mm bine to bring EFIG. 5, into coincidence with I'.',, as is indicated in FIG. 6. Varyingthe dial 96 produces a small current shown in FIG. 6, which shifts thephase of the current in accordance with Equation 16, producing a phasechange of a in E,;,. This in turn causes a phase shiftof a in E FIG. 5,until E is coincident in phase with E,, as indicated in FIG. 6. Then thedefect angle of the bushing fl=a is read on the potentiometer dial 96and the capacitance C, is read on dial 88.

In FIG. 7 is shown a second circuit connection of connectors 40-42. Inthis embodiment, resistor 80 is shorted by connecting jumper 150 acrossconnectors 40 and 42. Partial discharges, originating in the insulatingmaterial, create current pulses which, coupled through the low impedanceof the power system, produce voltages across impedor 81. The voltagescontain certain frequencies which are accentuated by the circuitparameters, including the impedance characteristics of impedor 81,connecting leads 158 and 160, and the input circuit of the amplifier, aswell as the C and C capacitances of the bushing. A suitable method ofdetecting such voltages is to employ a wide band (e.g., 20 kHz tolMl-Iz) amplifier. The lower limit of the band is selected to be wellabove any significant harmonics of the power-system frequency. A typicalvalue of impedor 81 is 5-10 millihenries. The amplifier therefore alsoacts as a filter and the output signal on line 162 will be substantiallyfree of the power frequency component. The signal is then observed on asuitable measurement and detecting device, for example the cathode-rayoscilloscope 164. Other detection devices such as a peak reading meteror a combination of highpass filter, rectifier, and d-c meter may alsobe used.

Another embodiment of the invention is shown in FIG. 8 in which theimpedor Z consists of a fixed capacitor 130 with negligible defectangle. As with the capacitor 34 of FIG. 1 and the resistor 80 of FIG. 4,it is connected permanently between the capacitance tap 32 and ground22. The reactance of capacitor 130 is so small compared with that ofcapacitance C; of the bushing that the portion of the bushing current I,that flows through C is negligible compared with that which flows fromthe capacitance tap 32 to the capacitor 130 and the external measuringsystem. For example, if the capacitance of 130 is 27 pf, at a frequencyof 60 Hz, its reactance is 88 ohms which is negligible as compared withthe reactance of l megohm of C As with the impedor Z of FIGS. 1 and 4,the low value of the reactance of capacitor 130 combined with the factthat it is connected permanently between the capacitance tap 32 andground 22 limits the voltage of the capacitance tap 32 to the safe valueof only a few volts.

As in FIGS. 1 and 4, the terminals of the impedor 130 are connected tothe external measuring circuit by means of leads 36 and 38 and testconnectors 40 and 42 which preferably consist of a polarized plugreceptacle. An adjustable capacitor 132 of negligible defect angle, andpreferably with a dial scale, is connected between the two leads 44 and46 of the external measuring system, thus connecting the capacitor 132in parallel with capacitor 130 when the connection has been made. Theupper lead 44 connects with the negative terminal 136 of amplifier 84'in series with capacitor 134. The system connections of FIG. 8 fromamplifier 84' and beyond correspond with those of FIG. 4 as a study ofthe two Figures shows.

As in Equation l the current in bushing 10,

I,=E,,u. C Z90a l which is shown in the phasor diagram, FIG. 9.

The current I in the bushing 10 divides at capacitance tap 32, aninsignificant amount flowing to ground through C and the two components1 and 1, flow to ground through capacitors 130 and 132 respectively. Theremainder of the current 1, flows through capacitor 134 to the negativeterminal 136 of amplifier 84. Since the negative terminal of amplifier84' is essentially at ground potential, C is equivalent to being inparallel with C and C Hence,

As the total impedance connected between capacitance tap 32 and ground22 consists of the capacitors C C 2, and C in parallel, or equivalent,the voltage E, between capacitance tap 32 and ground will lag I, by asshown in the phasor diagram of which corresponds to Equation l 5 I 114/+3 E r:

which corresponds to Equation (l7).

Substituting 1 from (28),

I Cm 1 0.

1 lso i m m n Comparing (32) with 18), it is apparent that theexpression C /(C +C, +C is equivalent to R /(R +R +R and accordingly canbe substituted for it in the Equations (19), and (22). Also, it followsthat Equation (23) will become A(C, +C, +C, and that the dial C willread in terms of the capacitance of the bushing.

The setting of the ratio dial 104 to the value K and the adjustment ofthe capacitance dial 132 and that of the defect angle dial 96 are madeas for FIG. 4,. When the null detector indicates a null reading, thevoltage E is in equality with E,

FIG. 9 is the phasor diagram for the system of FIG. 8 after the voltageE has been brought into equality with E and resembles FIG. 6. Thevoltage E across the capacitor now lags 1 by 90. The adjustment of thepotentiometer 96 now introduces a small current 1, producing the angle[-3 in accordance with Equation l6).

In the embodiments shown in FIGS. 4 and 8 the magnitude of the voltage Edeveloped across the impedor Z is amplified, shifted in phase and itsmagnitude adjusted, until it is brought into equality with a referencevoltage E, In the first embodiment (FIG. 1) the impedor voltage E ismerely amplified without change of phase. and is then compared with thereference voltage E. by means of a potentiometer and a high inputimpedance voltmeter. When the contactor of the potentiometer is adjusteduntil the reading of the voltmeter becomes a minimum, the power factorof the bushing and its capacitance are then accurately measured. In

been shown and described, various modifications thereof will be apparentto those skilled in the art and therefore it is not intended that theinvention be limited to the disclosed embodiments or todetails thereof,the departures may be made therefrom within the spirit and scope of theinvention.

What is claimed is: i

1. Apparatus for measuring defect characteristics of a bushing whilesaid bushing is in service connected to a high voltage source, saidbushing having agrounded flange, a high voltage electrode passingthroughsaid grounded flange, sheet insulating material and metallic foilinterwrapped around said' electrode and a capacitance tap connection toa layer of said metallic foil near the outer wrap, of said insulatingmaterial which capacitance tapconnection is brought out to a terminalnear said flange, comprising:

impedor means connected permanently between said capacitance tapterminal of said bushing and ground,

the impedance of said impedor means being so low relative to that ofsaid sheet insulating material and to that of said bushing betweensaidcapacitance tap terminal and ground that the current in said impedormeans is determined almost solely by the impedance of said sheetinsulating material and that the voltage between said capacitance tapterminal and ground never exceeds a value of one hundred volts,connector means permanently connected to said impedor means to permitthe coupling of an external circuit to said impedor means withoutinterruption of the current through said sheet insulating material, and

an external circuit adapted to be coupled to said impedor means andresponsive to a signal applied to said impedor means from said bushingfor producing an output indicative of a bushing defect characteristic,said connector means permitting said external circuit to be detachablycoupled to said impedor means.

2. Apparatus as claimed in claim 1 wherein said impedance of saidimpedor means is so low that substan tially the entire current flowingthrough said sheet insulating material also flows through said impedormeans,

said impedor means forming an element of said external circuit designedto measure the capacitance and defect angle of said bushing, and whereinsaid external circuit includes detachable connecting means to permit thecoupling of said external circuit to said impedor means withoutinterruption of the current through said sheet insulating material,

a first control to adjust the output voltage of said im pedor inaccordance with said capacitance of said bushing,

amplifying means to increase the output voltage of said first control,means to shift the phase of the output of said amplifying means,

magnitude control means to adjust the magnitude of the output voltage offirst control, means to ampli-.

" fy the output voltage of said first control to produce an outputvoltage for comparison with a reference voltage, a comparison means forcomparing said output voltage with said reference voltage, and

lead means for conducting said reference voltage to i said comparisonmeans.

3. Apparatus according to claim 2 wherein said impedor means is a fixedcapacitor responsive to the current flowing therethrough to develop avoltage across said capacitor,

said amplifying means comprises a' high gain operational amplifierhaving an input terminal and an output terminal and being arranged toamplify said voltage across said capacitor to produce said outputvoltage, and said magnitude control means is an adjustable potentiometerdefining a feedback loop between said output terminal and said inputterminal to provide control over the magnitude of said gain and ofsaidoutput voltage by adjustment of said potentiometer.

4. Apparatus as claimed in claim 3 wherein said comparison meanscomprises a second potentiometer arranged to be adjusted to vary themagnitude of said reference voltage conducted for comparison by saidlong shielded leads and having an output for comparison to said outputvoltage, and

a voltmeter arranged to measure the difference between said outputvoltage and said output for comparison.

5. Apparatus as claimed in claim 4 wherein said second potentiometer iscalibrated in terms of said capacitance of said bushing and is arrangedto indicate said capacitance when said difference indicated by saidvoltmeter attains a minimum value.

6. Apparatus as claimed in claim 1 wherein the impedance of said impedormeans is so low that substantially the entire current flowing throughsaid sheet insulating material also flows through said impedor means,

said impedor means forming an element of said external circuit designedto measure said capacitance and defect angle of said bushing, and

wherein said external circuit includes detachable connecting means topermit the coupling of said external circuit to said impedor meanswithout interruption of the current through said sheet insulatingmaterial,

a first control calibrated in terms of said capacitance of said bushing,arranged to indicate said capacitance when the output voltage of saidmeasuring system is in balance with a reference voltage,

first amplifying means to augment the voltage output of said firstcontrol, means to shift the phase of the output voltage of said firstamplifying means in conformity with the phase of said reference voltage,

magnitude control means to adjust the ratio of the voltage at said highvoltage electrode with said reference voltage,

second amplifying means to increase the voltage output of said magnitudecontrol means to produce a system output voltage,

comparison means for comparing said output voltage with said referencevoltage, and

lead means for conducting said reference voltage to said comparisonmeans.

7. Apparatus as claimed in claim 6 wherein said first amplifier is anoperational amplifier having input and output terminals, said impedor ispredominately resistive and of known magnitude, and said magnitudecontrol comprises an adjustable potentiometer connected between saidoutput terminal and ground and having its arm connected to a feedbackcircuit of said operational amplifier.

8. Apparatus as claimed in claim 2 wherein said impedor means is a fixedcapacitor having a low reactance and said external circuit hasadjustable capacitive control means arranged to be connected in parallelwith said fixed capacitor.

9. Apparatus as claimed in claim 8 wherein said amplifying means has anoutput terminal, said adjustable capacitive control comprises said firstcontrol and said second control is an adjustable potentiometer connectedbetween said output terminal and ground and having its arm connected toa feedback circuit of said operational amplifier.

10. Apparatus as claimed in claim 1 wherein said impedor means includesan inductor and a second impedor, said second impedor adapted to form anelement of a measuring system designed to measure said capacitance anddefect angle of said bushing, and said external circuit further includesindicating means responsive to high frequency signals produced bypartial discharge phenomena and coupled by said inductor to provideindications of voids in said insulating material in said bushing.

1 1. Apparatus for measuring defect characteristics of a bushing havinga high voltage electrode passing through a grounded flange, insulatingmaterial around said electrode and a capacitance tap, while said bushingis in service connected to a high voltage source, comprising:

impedor means connected permanently between said capacitance tap of saidbushing and ground,

the impedance of said impedor means being so low relative to that ofsaid insulating material and to that of said bushing between saidcapacitance tap and ground that the current in said impedor means isdetermined almost solely by the impedance of said insulating materialand that the voltage between said capacitance tap and ground neverexceeds a value of l volts, said impedor means including an inductor anda second impedor, said second impedor adapted to form an element of ameasuring system designed to measure said capacitance and defect angleof said bushing, and external circuit means comprising detachableconnecting means to permit the alternate coupling of first and secondexternal circuits to said inductive impedor and said second impedorwithout interruption of the current through said insulating material,said first external circuit includin g means responsive substantiallyonly to frequencies substantially above the frequency of said highvoltage source for producing output indicative of bushing partialdischarge,

said second external circuit including transducing means for producingan output voltage as a function of the current flowing through saidsecond impedor,

phase shifting means to adjust the phase of said output voltage inconformity with the phase of said reference voltage,

magnitude control means to adjust the magnitude of said output voltagein conformity with the magnitude of said reference voltage,

comparison means for comparing said output voltage with a referencevoltage, and a lead means for conducting said reference and outputvoltages to said comparison means.

12. Apparatus as claimed in claim 11 wherein said second externalcircuit further includes a first control associated with said magnitudecontrol means to adjust the magnitude of said output voltage, said firstcontrol being calibrated in terms of the capacitance of said bushing andsaid known ratio of said low reference voltage to said high voltage, and

a second control associated with said phase shifting means to adjust thephase angle of said output voltage, said second control being calibratedin terms of the power factor of said bushing.

13. Apparatus as'claimed in claim 12 wherein said transducing meanscomprises an operational amplifier.

14. Apparatus as claimed in claim 13 in which said second impedor ispredominately resistive and of known magnitude.

15. Apparatus as claimed in claim 14 wherein said operational amplifierhas input and output terminals, and said magnitude control comprises anadjustable potentiometer connected between said output terminal andground andhaving its arm connected to a feedback circuit of saidoperational amplifier.

16. Apparatus as claimed in claim 15 wherein said second impedor is afixed capacitor having a low reactance and said external circuit hasadjustable capacitive control means arranged to be connected in parallelwith said fixed capacitor.

17. Apparatus as claimed in claim 16 wherein said amplifier has anoutput terminal, said adjustable capacitive control comprises said firstcontrol and said second control is an adjustable potentiometer connectedbetween said output terminal and ground and having its arm connected toa feedback circuit of said operational amplifier.

18. Apparatus for indication of bushing defect characteristics whilesaid bushing is in service connected to a high voltage source comprisinga bushing having a high voltage electrode, a grounded flange throughwhich said high voltage electrode passes, insulating material aroundsaid electrode and a capacitance tap, impedor means connectedpermanently between said capacitance tap of said bushing and ground,said impedor means including an inductive impedor of low reactanceproducing a voltage in response to current flowing therethrough, and asecond impedor connected in series with said inductive impedor, theimpedance of said impedor means being so low relative to that of saidinsulating material and to that of said bushing between said capacitancetap and ground that the current in said impedor means is determinedalmost solely by the impedance of said insulating material and that thevoltage between said capacitance tap and ground never exceeds a value ofvolts, and means for coupling an external circuit to said impedor meansfor measurement of defect characteristics of said bushing, saidproducing an output indicative of bushing capacitance and defect angle.

19. Apparatus as claimed in claim 18 and further including polarizedconnector means permanently connected to said impedor means forpermitting said external circuit to be detachably coupled to saidimpedor means.

1. Apparatus for measuring defect characteristics of a bushing whilesaid bushing is in service connected to a high voltage source, saidbushing having a grounded flange, a high voltage electrode passingthrough said grounded flange, sheet insulating material and metallicfoil interwrapped around said electrode and a capacitance tap connectionto a layer of said metallic foil near the outer wrap of said insulatingmaterial which capacitance tap connection is brought out to a terminalnear said flange, comprising: impedor means connected permanentlybetween said capacitance tap terminal of said bushing and ground, theimpedance of said impedor means being so low relative to that of saidsheet insulating material and to that of said bushing between saidcapacitance tap terminal and ground that the current in said impedormeans is determined almost solely by the impedance of said sheetinsulating material and that the voltage between said capacitance tapterminal and ground never exceeds a value of one hundred volts,connector means permanently connected to said impedor means to permitthe coupling of an external circuit to said impedor means withoutinterruption of the current through said sheet insulating material, andan external circuit adapted to be couplEd to said impedor means andresponsive to a signal applied to said impedor means from said bushingfor producing an output indicative of a bushing defect characteristic,said connector means permitting said external circuit to be detachablycoupled to said impedor means.
 2. Apparatus as claimed in claim 1wherein said impedance of said impedor means is so low thatsubstantially the entire current flowing through said sheet insulatingmaterial also flows through said impedor means, said impedor meansforming an element of said external circuit designed to measure thecapacitance and defect angle of said bushing, and wherein said externalcircuit includes detachable connecting means to permit the coupling ofsaid external circuit to said impedor means without interruption of thecurrent through said sheet insulating material, a first control toadjust the output voltage of said impedor in accordance with saidcapacitance of said bushing, amplifying means to increase the outputvoltage of said first control, means to shift the phase of the output ofsaid amplifying means, magnitude control means to adjust the magnitudeof the output voltage of first control, means to amplify the outputvoltage of said first control to produce an output voltage forcomparison with a reference voltage, comparison means for comparing saidoutput voltage with said reference voltage, and lead means forconducting said reference voltage to said comparison means.
 3. Apparatusaccording to claim 2 wherein said impedor means is a fixed capacitorresponsive to the current flowing therethrough to develop a voltageacross said capacitor, said amplifying means comprises a high gainoperational amplifier having an input terminal and an output terminaland being arranged to amplify said voltage across said capacitor toproduce said output voltage, and said magnitude control means is anadjustable potentiometer defining a feedback loop between said outputterminal and said input terminal to provide control over the magnitudeof said gain and of said output voltage by adjustment of saidpotentiometer.
 4. Apparatus as claimed in claim 3 wherein saidcomparison means comprises a second potentiometer arranged to beadjusted to vary the magnitude of said reference voltage conducted forcomparison by said long shielded leads and having an output forcomparison to said output voltage, and a voltmeter arranged to measurethe difference between said output voltage and said output forcomparison.
 5. Apparatus as claimed in claim 4 wherein said secondpotentiometer is calibrated in terms of said capacitance of said bushingand is arranged to indicate said capacitance when said differenceindicated by said voltmeter attains a minimum value.
 6. Apparatus asclaimed in claim 1 wherein the impedance of said impedor means is so lowthat substantially the entire current flowing through said sheetinsulating material also flows through said impedor means, said impedormeans forming an element of said external circuit designed to measuresaid capacitance and defect angle of said bushing, and wherein saidexternal circuit includes detachable connecting means to permit thecoupling of said external circuit to said impedor means withoutinterruption of the current through said sheet insulating material, afirst control calibrated in terms of said capacitance of said bushing,arranged to indicate said capacitance when the output voltage of saidmeasuring system is in balance with a reference voltage, firstamplifying means to augment the voltage output of said first control,means to shift the phase of the output voltage of said first amplifyingmeans in conformity with the phase of said reference voltage, magnitudecontrol means to adjust the ratio of the voltage at said high voltageelectrode with said reference voltage, second amplifying means toincrease the voltage output of said magnitude control means to produce asystem output volTage, comparison means for comparing said outputvoltage with said reference voltage, and lead means for conducting saidreference voltage to said comparison means.
 7. Apparatus as claimed inclaim 6 wherein said first amplifier is an operational amplifier havinginput and output terminals, said impedor is predominately resistive andof known magnitude, and said magnitude control comprises an adjustablepotentiometer connected between said output terminal and ground andhaving its arm connected to a feedback circuit of said operationalamplifier.
 8. Apparatus as claimed in claim 2 wherein said impedor meansis a fixed capacitor having a low reactance and said external circuithas adjustable capacitive control means arranged to be connected inparallel with said fixed capacitor.
 9. Apparatus as claimed in claim 8wherein said amplifying means has an output terminal, said adjustablecapacitive control comprises said first control and said second controlis an adjustable potentiometer connected between said output terminaland ground and having its arm connected to a feedback circuit of saidoperational amplifier.
 10. Apparatus as claimed in claim 1 wherein saidimpedor means includes an inductor and a second impedor, said secondimpedor adapted to form an element of a measuring system designed tomeasure said capacitance and defect angle of said bushing, and saidexternal circuit further includes indicating means responsive to highfrequency signals produced by partial discharge phenomena and coupled bysaid inductor to provide indications of voids in said insulatingmaterial in said bushing.
 11. Apparatus for measuring defectcharacteristics of a bushing having a high voltage electrode passingthrough a grounded flange, insulating material around said electrode anda capacitance tap, while said bushing is in service connected to a highvoltage source, comprising: impedor means connected permanently betweensaid capacitance tap of said bushing and ground, the impedance of saidimpedor means being so low relative to that of said insulating materialand to that of said bushing between said capacitance tap and ground thatthe current in said impedor means is determined almost solely by theimpedance of said insulating material and that the voltage between saidcapacitance tap and ground never exceeds a value of 100 volts, saidimpedor means including an inductor and a second impedor, said secondimpedor adapted to form an element of a measuring system designed tomeasure said capacitance and defect angle of said bushing, and externalcircuit means comprising detachable connecting means to permit thealternate coupling of first and second external circuits to saidinductive impedor and said second impedor without interruption of thecurrent through said insulating material, said first external circuitincluding means responsive substantially only to frequenciessubstantially above the frequency of said high voltage source forproducing output indicative of bushing partial discharge, said secondexternal circuit including transducing means for producing an outputvoltage as a function of the current flowing through said secondimpedor, phase shifting means to adjust the phase of said output voltagein conformity with the phase of said reference voltage, magnitudecontrol means to adjust the magnitude of said output voltage inconformity with the magnitude of said reference voltage, comparisonmeans for comparing said output voltage with a reference voltage, andlead means for conducting said reference and output voltages to saidcomparison means.
 12. Apparatus as claimed in claim 11 wherein saidsecond external circuit further includes a first control associated withsaid magnitude control means to adjust the magnitude of said outputvoltage, said first control being calibrated in terms of the capacitanceof said bushing and said known ratio of said low reference voltage tosaid high voltage, and a second control associated with said phaseshifting means to adjust the phase angle of said output voltage, saidsecond control being calibrated in terms of the power factor of saidbushing.
 13. Apparatus as claimed in claim 12 wherein said transducingmeans comprises an operational amplifier.
 14. Apparatus as claimed inclaim 13 in which said second impedor is predominately resistive and ofknown magnitude.
 15. Apparatus as claimed in claim 14 wherein saidoperational amplifier has input and output terminals, and said magnitudecontrol comprises an adjustable potentiometer connected between saidoutput terminal and ground and having its arm connected to a feedbackcircuit of said operational amplifier.
 16. Apparatus as claimed in claim15 wherein said second impedor is a fixed capacitor having a lowreactance and said external circuit has adjustable capacitive controlmeans arranged to be connected in parallel with said fixed capacitor.17. Apparatus as claimed in claim 16 wherein said amplifier has anoutput terminal, said adjustable capacitive control comprises said firstcontrol and said second control is an adjustable potentiometer connectedbetween said output terminal and ground and having its arm connected toa feedback circuit of said operational amplifier.
 18. Apparatus forindication of bushing defect characteristics while said bushing is inservice connected to a high voltage source comprising a bushing having ahigh voltage electrode, a grounded flange through which said highvoltage electrode passes, insulating material around said electrode anda capacitance tap, impedor means connected permanently between saidcapacitance tap of said bushing and ground, said impedor means includingan inductive impedor of low reactance producing a voltage in response tocurrent flowing therethrough, and a second impedor connected in serieswith said inductive impedor, the impedance of said impedor means beingso low relative to that of said insulating material and to that of saidbushing between said capacitance tap and ground that the current in saidimpedor means is determined almost solely by the impedance of saidinsulating material and that the voltage between said capacitance tapand ground never exceeds a value of 100 volts, and means for coupling anexternal circuit to said impedor means for measurement of defectcharacteristics of said bushing, said coupling means enabling thealternate coupling of said inductive reactance to a first externalcircuit substantially responsive only to frequencies above that of thehigh voltage source for producing an output indicator of bushing partialdischarge, and said second impedor to a second external circuitresponsive to a low reference voltage having a known ratio to said highvoltage source and a known phase relation thereto for producing anoutput indicative of bushing capacitance and defect angle.
 19. Apparatusas claimed in claim 18 and further including polarized connector meanspermanently connected to said impedor means for permitting said externalcircuit to be detachably coupled to said impedor means.